A. Field of the Invention
This invention relates to surface controlled subsurface safety valves utilized to control flow at a subsurface location in a well.
B. The Prior Art
Well pressure may assist closure of a surface controlled subsurface safety valve as disclosed in U.S. Pat. No. 3,703,193 to Raulins. However, as well pressure approaches zero such a safety valve becomes depth sensitive. As well pressure approaches zero, spring force, or other inherent resilient urging means, is relied upon to close the valve.
Some surface controlled subsurface safety valves, such as disclosed in U.S. Pat. No. 3,696,868 to Taylor, include a balance pressure chamber. Valve closure of such a safety valve may be assisted by pressurizing the balance pressure chamber. However, heretofore, designing a subsurface safety valve having a balance pressure chamber so that valve closure may be assisted by well pressure has resulted in increasing the depth sensitive limitations of the valve as the balance pressure approaches zero, vis a vis a valve without a balance pressure chamber. Also, heretofore, designing a subsurface safety valve having a balance pressure chamber so that a greater force tending to close the valve is produced when there is approximately the same pressure within both the balance pressure chamber and the control pressure chamber has resulted in well pressure tending to maintain the valve open.